1. Field of the Invention
The present invention relates to a wiring board for flip-chip-mounting. More specifically, the invention relates to a wiring board that has a very flat surface and that is used for flip-chip-mounting a semiconductor element (hereinafter often called peripheral-type semiconductor element), which is very flat and has flip- chip-connect electrodes arranged in a circular manner along the peripheral edge on the lower surface thereof.
2. Description of the Prior Art
So far, a ceramic wiring board obtained by forming many wiring conductors of a high-melting metal such as tungsten, molybdenum or manganese on the surface, on the inside or on the bottom surface of an insulating board composed of a ceramic such as a sintered product of alumina, has been most widely used for a package containing a semiconductor element or hybrid integrated circuit devices on which are mounted a semiconductor element as well as various electronic parts such as capacitors and resistors. In the above-mentioned package, for instance, a surface region (e.g., recessed portion) for mounting a semiconductor element is formed in nearly the central portion in the upper surface of the ceramic insulating board, and a semiconductor element is secured onto the surface region by using an adhesive such as a glass, a resin or a brazing material. The electrodes formed on the semiconductor element are electrically connected to the wiring conductors formed surrounding the surface region by bonding the wires. Moreover, a closure made of a metal or a ceramic is secured, by using the same adhesive as the one mentioned above, onto the surface region of the insulating board in order to air-tightly seal the semiconductor element.
Accompanying the trend toward designing the semiconductor elements such as ICs and LSIs in a highly integrated form and in very small sizes yet so as to operate at high speeds, in recent years, however, the wiring conductors formed on the wiring board for mounting the semiconductor element are becoming ever fine and are forming highly dense wiring patterns, and it is becoming necessary to mount the semiconductor element more compactly. Because of this reason, a flip-chip-mounting method has been employed more frequently to directly connect the electrodes of the semiconductor element to the wiring conductors of the insulating board (wiring board) by using solder bumps instead of bonding the wires. That is, according to the flip-chip-mounting method, the electrodes of the semiconductor element are directly connected to the wiring conductors and, hence, the region on where the semiconductor element is to be mounted on the surface of the insulating board must be flat to a high degree.
The electronic devices having a wiring board on which the semiconductor element is mounted are finding expanding applications, and are being used in a variety of environments under strict conditions. For example, many of the vehicles such as automobiles have been furnished with electronic control equipment, and the electronic devices mounted on the vehicles are used under very strict environmental conditions and must, hence, be highly reliable. Factors which are most important for maintaining a high reliability under severe environmental conditions are mechanical properties such as shearing strength or thermal fatigue life in a portion where the circuit is joined by soldering. According to the above-mentioned flip-chip-mounting method, the portions where the semiconductor element is joined to the wiring conductors on the surface of the insulating board relying on the solder bumps, exhibit excellent shearing strength and thermal fatigue life playing an important role for maintaining reliability of the electronic devices that are used being mounted on the vehicles. From this point of view, too, the wiring board on which the semiconductor element is flip-chip-mounted must have a very flat surface in a region on where the semiconductor element is mounted.
According to the conventional wiring board, however, the electrically insulating ceramics constituting the insulating board has a coefficient of thermal expansion greatly different from that of the wiring conductors, making it difficult to maintain a high degree of flatness.
That is, the above wiring board is prepared by applying an electrically conducting paste, according to a predetermined wiring pattern, onto a ceramic green sheet having through-holes formed at predetermined positions, filling the through-holes with the electrically conducting paste and, as required, laminating these green sheets one upon the other, followed by simultaneously firing the green sheets and the electrically conducting paste. Due to a difference in the contraction between the green sheet and the electrically conducting paste during the step of firing, however, the wiring board (insulating board) is warped or undulated, making it difficult to obtain a highly flat wiring board maintaining a good yield.
In order to solve this problem, Japanese Examined Patent Publication (Kokoku) No. 25277/1990 and Japanese Unexamined Patent Publication (Kokai) No. 31368/1992 disclose methods of preparing a wiring board by simultaneously firing a ceramic green sheet and an electrically conducting paste, and firing them again while applying a uniform load by using a suitable jig.
According to such a method, however, the firing must be repeated a plural number of times and the load must be uniformly exerted by using a jig or the like, requiring cumbersome operations, resulting in an increase in the cost of production and greatly deteriorating the production efficiency. Besides, the surface state of the jig used for exerting the load is transferred onto the surface of the insulating board, causing the flatness of the insulating board to be deteriorated. Besides, the wiring pattern formed by the electrically conducting paste is broken or is short-circuited due to the contact to the jig.